In the fabrication of a semiconductor micro circuit, fine patterns are formed in which pattern shapes drawn on a mask are transferred to a photosensitive resin (a resist) applied on the surface of a sample using a light source such as an ArF excimer laser. This is called a lithography process. For measurement (length measurement) for the dimension management of resist patterns formed in the lithography process, a scanning electron microscope (SEM) is generally used. The SEM is a device in which an electron beam emitted from an electron source is focused on the surface of a sample through an electron lens formed by a magnetic field or an electric field, the electron beam is deflected using a magnetic field or an electric field to scan the surface of the sample, and the electron beam is applied to detect secondary electrons emitted from the sample. Since the amount of secondary electrons to be emitted depends on the structure of the surface of the sample, the amounts of secondary electrons to be emitted for the individual application positions are displayed as an image, and the contrast reflecting the structure and material of the sample can be obtained. This image is called an SEM image. Since the electron beam can be focused as small as the order of nanometers, dimensional variations on the order of nanometers can be measured using the SEM, and the dimension management of fine patterns can be performed.
However, when fine resist patterns are observed using the SEM, a problem arises in that a resist is contracted (shrunk) caused by the application of an electron beam. Since the resist is shrunk, the pattern dimensions on an SEM image become smaller than the dimensions before SEM observation. On this account, an error is taken place on the value of length measurement. For a method for decreasing shrinkage, Patent Literature 1 discloses a method in which in the application of an electron beam, a scanning line spacing is set so as not to exceed an application density determined based on the physical properties of a sample.
Moreover, when such observation is performed that a scan area (a visual field) on the surface of a sample is a few tens microns or greater using an SEM, a problem arises in that a large aberration (deflection aberration) is taken place, which is caused by the deflection of an electron beam, and a large image blur or image distortion is taken place at the end of the visual field. For a method for decreasing an image blur or image distortion as described above, Patent Literature 2 discloses a method in which deflectors in two stages are used and the strengths and directions of the deflection magnetic fields produced in the deflectors are set to predetermined values. Furthermore, Patent Literature 3 discloses a method in which the deflection sensitivity and deflection direction of deflectors in two stages are linked to the size of a visual field for control.
In addition, Patent Literature 4 discloses a method in which a focal point or an astigmatic point is adjusted according to scan positions, and an image field curvature aberration and an astigmatism in deflection aberrations are corrected to decrease image blurs. However, the response speeds of a corrector using magnetic fields and the control circuit of the corrector, which are mounted on a typical SEM are slower than an adjusting method using electric fields, and it is not enabled to realize changing correction conditions at high speed. In other words, it is necessary to provide a method for decreasing deflection aberrations, which does not need high-speed focus correction or astigmatism control.